This invention relates generally to color cathode ray picture tubes and is addressed particularly to a front assembly for use in the manufacture of color tubes that have a tension foil shadow mask. The invention is applicable to the manufacture of tension mask tubes of various types including those intended for home entertainment television receivers and medium-resolution and high-resolution tubes for color monitors.
A color cathode ray tube typically has three electron guns arranged in a delta or an in-line configuration. Each gun projects an electron beam through the apertures of a "shadow mask" onto assigned target areas located on the inner surface of the faceplate. The target areas comprise patterns of phosphor deposits typically arranged in triads of dots or lines. Each of the triads consists of a deposit of a red-light-emitting, green-light-emitting, and blue-light-emitting phosphor which are excited to luminescence under bombardment by the respective electron beams. To increase the apparent brightness of the display, and to minimize the incidence of color impurities that can result if a beam falls upon an unassigned phosphor deposit, the target area may include a layer of darkish, light-absorbing material termed a "grille," and its composition is referred to as "grille dag." The grille dag separates the dots or lines, and serves as a guard band in case of beam mis-registration. The grille dag is also electrically conductive, unlike the phosphor deposits.
The front assembly of a color cathode ray tube essentially comprises the faceplate with its deposits of the grille dag and the light-emitting phosphors, a shadow mask, and a support structure for the mask. As is well known in the art, the shadow mask acts as a color-selection electrode, or parallax barrier, that ensures that each of the three beams lands only on its assigned phosphor deposits. As used herein, the term shadow mask means the "tension foil shadow mask," which comprises an apertured metallic foil which may, by way of example, be about one mil or less in thickness. This type of mask must be supported in high tension a predetermined distance from the inner surface of the cathode ray tube faceplate; this distance is known as the "Q"-distance.
The physical requirements for the tension foil shadow mask support structure are stringent. As the shadow mask is mounted under high tension, the structure must be of high strength so that the mask is held immovable--an inward movement of the mask as little as one-tenth of a mil can result in loss of guard band and consequent color impurities. Also, the mask support structure must be of such configuration and material composition as to be compatible with the means to which it is secured. For example, the mask support structure is attached to glass such as the glass of the inner surface of the faceplate, so it is essential that the material from which the structure is made have about the same thermal coefficient of expansion as that of the glass so the glass will not crack as a result of thermal stress. Also, the mask support structure must be of such composition that the mask can be securely fastened to it, as by electrical resistance or laser welding, for example. And the mask support surface is preferably of such flatness as to minimize voids between the metal of the mask and the mask support structure which might prevent the intimate metal-to-metal contact required for welding.
As noted, the screen area receives process coating materials in the form of the grille and the light-emitting phosphors. These materials are typically deposited by a photoprinting process. The screen area of the inner surface of the faceplate is first coated with a fluidized material that constitutes the grille, and which hardens upon drying. The shadow mask, mounted on a rigid frame, is temporarily installed in precise relationship to the faceplate, and the grille coating is exposed to light actinic to the coating projected through the apertures of the mask from a light source located at a position that corresponds to the beam-emission point of the associated electron gun of the end-product tube. The faceplate is then separated from the shadow mask and the coating is "developed," resulting in a pattern of openings in the grille. The light-emitting phosphors are then sequentially deposited in respective grille openings by a repetition of the same process. The final product is a faceplate having on its inner surface a pattern of triads of dots or lines capable of emitting, upon electron beam excitation, red, green or blue light.
The deposits on the inner surface of the faceplate of the cathode ray tube are typically covered with a conductive film of aluminum. The aluminizing process comprises the deposition of an electron-pervious metallic film; that is, a film trasnparent to the flow of electrons comprising the three beams. The film increases the brightness of the display by acting as a mirror to reflect toward the viewer the visible light produced by the phosphors when activated by the beams. The film also carries the high-voltage charge to act as an electron-attractive ultor electrode for the display. The thickness of the film is typically about 2,000 Angstroms.
The process coating materials such as the grille coating and the phosphors for each color are typically applied in the form of screening fluids commonly referred to as "slurries." A slurry is conventionally applied to the screen area by a process known as the "radial flow suffusion," or as it is referred to herein, the "spin-application process." In this process, the screening fluid is poured onto the faceplate as it is rotated. The fluid spreads to the edges of the panel under the influence of centrifugal force, and excess fuid is cast off from the faceplate perimeter. If there is any obstacle to the free flow of the slurry during the spin-application process, the out-rushing slurry will "wash back" from the obstacle, resulting in thickened wave patterns in the coating which become fixed upon drying. The effect of the wave patterns is a non-uniformity in phosphor density thickness that can become cumulative as the process coating materials are successively applied. The presence of such wave patterns results in discontinuities in coating thickness visible to the viewer as dark areas on the screen. Also, underexposure of the coating in thickened areas during the photoscreening process can result in non-adherence of the phosphor and consequent phosphor wash-off and flake-off.
U.S. Pat. No. 3,894,321 to Moore, of common ownership herewith, is directed to a method for processing a color cathode ray tube having a foil mask sealed in tension directly to the bulb. Included in that disclosure is a description of the sealing of a foil mask between the junction of the skirt of the faceplate and the funnel. The mask is shown as having two or more alignment holes near the corners of the mask which mate with alignment nipples in the faceplate. The nipples pass through the alignment holes to fit into recesses in the funnel. In another Moore embodiment, the front panel is shown as having a continuous ledge around the inner surface of the faceplate. The top surface of the ledge is spaced a Q-distance away from the faceplate for receiving a foil mask such that the mask is sealed within the tube envelope. In yet another embodiment, two ledges are located at the sides of the faceplate parallel with the vertical axis of the faceplate for receiving a shadow mask. Also shown is an embodiment in which the faceplate is skirtless and essentially flat.
Other prior art: Strauss--4,547,696; Palac--4,100,451; Lerner--4,087,717; Schwartz--4,069,567; Dougherty--4,045,701; Steinberg et al--3,727,087; Oess--3,284,655; Hackett et al.--3,030,536; Vincent--2,905,845; Fischer-Colbrie---2,842,696; Law--2,625,734; and "The CBS Colortron: A color picture tube of advanced design." Fyler et al. Proc. of the IRE, Jan. 1954.